Currently developed or commercially available natural lighting systems (including sunlight condensing systems) are generally classified into stationary lighting systems using optical duct and sunlight condensing systems using a solar tracking lens (spherical lens or Fresnel lens).
A stationary lighting system using an optical duct has lower light condensing efficiency than a solar tracking type lighting system and is capable of lighting without significant influence of solar conditions (weather conditions). On the other hand, the solar tracking type lighting system enables lighting under conditions of clear sky or partially overcast sky and has high condensing efficiency.
It is difficult to determine superiority through simple comparison between these two kinds of lighting systems, and the stationary lighting systems are used for overall indoor illumination and the solar tracking type lighting systems are used for local indoor illumination in consideration of such merits and drawbacks.
In particular, the solar tracking type natural lighting systems are classified into a reflection mirror type (planar or curved reflector) and a lens type according to a principle of condensing light, or classified into a reflection mirror type and an optical fiber type according to a light transmission method.
The reflection mirror type enables transmission of sunlight over a long distance by a mirror without a separate light condensing unit, but requires a sufficient size of the mirror and a sufficient space for light transmission. The lens type has a limit to a light transmission distance (within 30 m) due to light transmission using optical fibers and low economic feasibility due to a limit of optical fibers.
Particularly, conventional natural lighting systems commonly suffer from spreading properties of condensed sunlight, which reduces luminous flux and makes light transmission difficult, so that significant optical loss occurs upon change of a direction during the light transmission, thereby making it difficult to achieve transmission of sunlight over a long distance.